Biologically Relevant Heterodinuclear Iron–Manganese Complexes

The heterodinuclear complexes [FeIIIMnII(L-Bn)­(μ-OAc)2]­(ClO4)2 (1) and [FeIIMnII(L-Bn)­(μ-OAc)2]­(ClO4) (2) with the unsymmetrical dinucleating ligand HL-Bn {[2-bis­[(2-pyridylmethyl)­aminomethyl]]-6-[benzyl-2-(pyridylmethyl)­aminomethyl]-4-methylphenol} were synthesized and characterized as biologically relevant models of the new Fe/Mn class of nonheme enzymes. Crystallographic studies have been completed on compound 1 and reveal an FeIIIMnIIμ-phenoxobis­(μ-carboxylato) core. A single location of the FeIII ion in 1 and of the FeII ion in 2 was demonstrated by Mössbauer and 1H NMR spectroscopies, respectively. An investigation of the temperature dependence of the magnetic susceptibility of 1 revealed a moderate antiferromagnetic interaction (J = 20 cm–1) between the high-spin FeIII and MnII ions in 1, which was confirmed by Mössbauer and electron paramagnetic resonance (EPR) studies. The electrochemical properties of complex 1 are described. A quasireversible electron transfer at −40 mV versus Ag/AgCl corresponding to the FeIIIMnII/FeIIMnII couple appears in the cyclic voltammogram. Thorough investigations of the Mössbauer and EPR signatures of complex 2 were performed. The analysis allowed evidencing of a weak antiferromagnetic interaction (J = 5.72 cm–1) within the FeIIMnII pair consistent with that deduced from magnetic susceptibility measurements (J = 6.8 cm–1). Owing to the similar value of the FeII zero-field splitting (DFe = 3.55 cm–1), the usual treatment within the strong exchange limit was precluded and a full analysis of the electronic structure of the ground state of complex 2 was developed. This situation is reminiscent of that found in many diiron and iron–manganese enzyme active sites.